Change includes of <cmath> to <boost/config/no_tr1/config.hpp>.
Previously if Boost.TR1 was in the include path then including <cmath> pulls in all the new TR1 math functions, which in turn also requires linking to an external library. With auto-linking support this requires that library to have been built and be present in the library search path, even if the actual library under use is header only.
Fixes #2392.